In modern integrated circuits, the material of choice for upper-level conductive interconnection systems has been aluminum, including doped aluminum and aluminum alloys. Aluminum is an attractive material for integrated circuit metallization due to its high conductivity and low cost. The processing required to form aluminum metallization is also relatively easy, as it can readily be evaporated or sputtered onto the wafer. Aluminum also is able to form good ohmic contact to both p-type and n-type doped semiconductor material, such as silicon. In addition, aluminum is quite compatible with conventional semiconductor processes, such as used to form bipolar and metal-oxide-semiconductor (MOS) devices, unlike other metals such as copper or gold which can diffuse into active regions and degrade device performance.
Certain drawbacks do exist for aluminum-based metallization systems, however, particularly as geometries enter the sub-micron regime. A well-known limitation of aluminum is its poor step coverage, particularly for vertical or retrograde sidewalls of contact openings through insulating layers such as silicon dioxide, and especially for sputtered aluminum, due to the shadowing effect of steep contact walls. In addition, mechanical stress in the aluminum film can cause voids therein. Stress-induced voids and step coverage faults of sufficient size can each cause an open in a metal line or contact. Furthermore, since aluminum metallization is subject to electromigration, and since the rate of electromigration increases with current density through the film, necking or narrowing of an aluminum line due to such voids or poor steps locally increase the current density thereat. As a result, the electromigration rate increases at a narrowed location of the film, greatly increasing the electromigration failure rate.
A prior technique for addressing these limitations of aluminum metallization systems includes the use of refractory metal plugs, such as tungsten plugs, to fill contact openings in insulating layers. According to one example of this technique (for which many specific methods are well known in the art), after the opening of contacts through the insulating layer, a layer of tungsten is deposited by CVD over the wafer in such a manner as to conformally fill the contact opening, and is subsequently etched back to expose the surface of the insulating layer with the tungsten remaining in the contact opening. Alternatively, selective tungsten deposition has been used (the tungsten depositing on silicon but not on silicon dioxide) to fill contact openings. In either case, a subsequently deposited aluminum layer can readily make contact to the tungsten plug.
While the tungsten plug technique has many advantages, including good step coverage in all contacts, compatibility with planarized processing, and tolerance of misalignment in the etch of overlying aluminum lines (since the aluminum can be etched selectively relative to the tungsten), the tungsten plug process adds complexity to the manufacturing flow. In addition, deposited tungsten is vulnerable to poor adhesion and high contact resistance, requiring the use of additional sputtered barrier films prior to the deposited tungsten, and the associated added process complexity therewith.
Our copending U.S. patent application Ser. No. 621,367, filed Nov. 30, 1990, assigned to SGS-Thomson Microelectronics, Inc., and incorporated herein by this reference, describes an aluminum plug process which addresses these limitations of both conventional aluminum films and tungsten plug processes. This process is also described in Chen, et al., "Planarized Aluminum Metallization for Sub-0.5 .mu.m CMOS Technology", IEDM Digest of Technical Papers, paper 3.4.1 (IEEE, Dec. 1990), pp. 51-54, also incorporated herein by this reference. According to this process, aluminum is sputtered at low enough rates, with the wafer at an elevated temperature, such that the sputtered aluminum migrates along the surfaces of contact openings to the bottom. Continued sputtering, either at the same or an increased rate, completely fills the contact opening, in effect forming an aluminum plug therein.
This process has provided dramatic results in forming submicron metallization with complete step coverage. In patterning and etching the overlying metallization lines, however, great care is preferably taken so that the edge of the overlying line does not lie within the contact opening. If the edge of the metal line overlies the contact, the metal line and plug may both be etched by the metal etch process all the way to the underlying layer, causing junction leakage and other degradation thereat. As a result, prior to the subject invention, the preferred layout of the metal lines included its widening at contact locations by a sufficient distance (referred to as the "enclosure") to compensate for the worst case misalignment. While this widening of the metal lines at the contact locations eliminates the problem of etching through the plug into the underlying contacted layer, the layout of the integrated circuit must include sufficient space for the enclosure. Particularly in regularly laid-out circuits such as memories, significant added chip size may result.
By way of further background, it is known to use insulating sidewall filaments to narrow contact openings, such that metal lines making contact therethrough need not be widened to provide misalignment tolerance. Such a technique is described in U.S. Pat. No. 4,656,732. However, as described in this reference, the insulating sidewalls also increase the series resistance of the contact, as the cross-sectional area of the ohmic contact is reduced (see column 5, line 64 through column 6, line 26) of the above-referenced U.S. Pat. No. 4,656,732. Such increase in the series resistance is of increasing concern as the contact openings become smaller, especially to sub-micron dimensions.
It is therefore an object of this invention to provide a method of forming an integrated circuit metallization system which eliminates the need for enclosure around contact locations.
It is a further object of this invention to provide such a method which allows the filling of contact openings with the same metallization as that of the metal lines.
It is a further object of this invention to provide such a method which fills contact openings with aluminum.
It is a further object of this invention to provide such a method which does not reduce the contact area and thus increase the contact resistance.
Other objects and advantages of this invention will be apparent to those of ordinary skill in the art having reference to the following specification, together with its drawings.